Communication devices such as User Equipments (UE) are also known as e.g. mobile terminals, wireless terminals and/or mobile stations. User equipments are enabled to communicate wirelessly in a cellular communications network or wireless communication system, sometimes also referred to as a cellular radio system or cellular networks. The communication may be performed e.g. between two user equipments, between a user equipment and a regular telephone and/or between a user equipment and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.
User equipments may further be referred to as mobile telephones, cellular telephones, or laptops with wireless capability, just to mention some further examples. The user equipments in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another user equipment or a server.
The cellular communications network covers a geographical area which is divided into cell areas, wherein each cell area being served by a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “eNB”, “NodeB”, “NodeB”, “B node”, or BTS (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro NodeB, home NodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the user equipments within range of the base stations.
In some RANs, several base stations may be connected, e.g. by landlines or microwave, to a radio network controller, e.g. a Radio Network Controller (RNC) in Universal Mobile Telecommunications System (UMTS), and/or to each other. The radio network controller, also sometimes termed a Base Station Controller (BSC) e.g. in GSM, may supervise and coordinate various activities of the plural base stations connected thereto. GSM is an abbreviation for Global System for Mobile Communications (originally: Groupe Spécial Mobile).
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
UMTS is a third generation mobile communication system, which evolved from the GSM, and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UMTS Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for user equipments. The 3GPP has undertaken to evolve further the UTRAN and GSM based radio access network technologies.
According to 3GPP/GERAN, a user equipment has a multi-slot class, which determines the maximum transfer rate in the uplink and downlink direction. GERAN is an abbreviation for GSM EDGE Radio Access Network. EDGE is further an abbreviation for Enhanced Data rates for GSM Evolution.
In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the mobile station. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the mobile station to the base station.
RACH is a common uplink transport channel used by a user equipment to access a network in case the user equipment does not have a dedicated uplink. The RACH channel may be used for transmitting data in Cell_FACH state. The RACH channel is often carrying signalling e.g. buffer-status measurements, cell update messages, etc, that will trigger a state change from Cell_FACH state to Cell_DCH state, where a dedicated uplink channel is available.
The CELL_DCH state is e.g. characterised by:                A dedicated physical channel is allocated to the user equipment in uplink and downlink.        The user equipment is known on cell level according to its current active set.        Downlink and uplink dedicated transport channels, downlink shared transport channels, and a combination of these transport channels can be used by the user equipment.        
The CELL_FACH state is e.g. characterised by:                No dedicated physical channel is allocated to the user equipment.        The user equipment monitors the downlink for transmissions.        The user equipment is assigned a default common or shared transport channel in the uplink, Random Access Channel (RACH) or Common Enhanced Dedicated channel (E-DCH), that it may use anytime according to the access procedure for that transport channel.        The position of the user equipment is known by UTRAN on cell level according to the cell where the user equipment last made a cell update.        
The RACH procedure comprises a power-ramping phase on the Physical Random Access Channel (PRACH), where the desire is to find the correct power level to be used by the user equipment for uplink transmission. Once a user equipment is detected on PRACH, the NodeB responds with an acquisition indication on the Acquisition Indication Channel (AICH), which acknowledges the correct power level on PRACH. The user equipment then continues by transmitting in-band RACH payload carried on PRACH.
In WCDMA the random access channel is divided in time into 750 access slots per second, and there are up to 16 different preambles that can be used per scrambling code. When making a random access attempt the user equipment chooses from a subset of the access slots and selects randomly one preamble out of a subset of the up to 16 possible preambles. After the preamble has been transmitted the user equipment waits for a response on the acquisition indicator channel (AICH). The timing of this response is determined by the parameter τp−a.
If no acknowledgement is received the random access attempt continues and a new preamble, randomly chosen from the available ones, is transmitted with higher power. This procedure is repeated until a response in the form of an acknowledgement or a non-acknowledgement is received, too many unsuccessful preamble transmissions have been made or the maximum allowed transmit power has been used by the user equipment for a number of transmissions. If the user equipment is not acknowledged during its random access attempt it will start another random access attempt at a later time. The stepwise preamble power increase is called power ramping. The idea is that random access user equipments should start their transmissions with low power to minimize the interference they generate, and with each new preamble that is transmitted increase the transmission power until it is high enough for successful reception by the NodeB. When the user equipment receives an acknowledgement it stops the power ramping and transmits the random access message with a power related to the one used for the last preamble.
An enhanced random access method known as “Enhanced Uplink in CELL_FACH state” was introduced in Release 8. The releases mentioned in this document relates to releases of HSPA 3GPP specifications. A portion of the preamble signatures may be set aside for Release 99 method and another portion for the Release 8 method. The preamble ramping procedure is essentially the same for both methods, except that an optional extended acquisition indication called Extended Acquisition Indication (EAI) has been introduced beside the ordinary AI. The message parts are more different for the two methods, showing similar differences as Release 99 Dedicated Channel (DCH) and Release 6 Enhanced Dedicated Channel (E-DCH) in the CELL_DCH state.
The problem with the existing solution is that different random access attempts might require different levels of receiver processing to be detected. Some preambles may be quickly detected and should then also be acknowledged quickly to keep latency down, while other preambles might require more advanced processing that requires longer processing time to enable successful detection.